1. Field of the Invention
The present application relates to methods and apparatus for the coding of speech signals; particularly (though not exclusively) to code excited linear predictive coding (LPC) in which input speech is analysed to derive the parameters of an appropriate time-varying synthesis filter, and to select from a "codebook" of excitation signals those which, when (after appropriate scaling) supplied in succession to such a synthesis filter, produce the best approximation to the original speech. The filter parameters, codeword identifying codebook entries, and gains, can be sent to a receiver where they are used to synthesise received speech.
2. Related Art
Commonly in such systems a long-term predictor is employed in addition to the LPC filter. This is best illustrated by reference to FIG. 1 of the accompanying drawings, which shows a block diagram of a decoder. The coded signal includes a codeword identifying one of a number of stored excitation pulse sequences and a gain value; the codeword is employed at the decoder to read out the identified sequence from a codebook store 1, which is then multiplied by the gain value in a multiplier 2. Rather than being used directly to drive a synthesis filter, this signal is then added in an adder 3 to a predicted signal to form the desired composition excitation signal. The predicted signal is obtained by feeding back past values of the composite excitation via a variable delay line 4 and a multiplier 5, controlled by a delay parameter and further gain value included in the coded signal. Finally the composite excitation drives an LPC filter 6 having variable coefficients. The rationale behind the use of the long term predictor is to exploit the inherent periodicity of the required excitation (at least during voiced speech); an earlier portion of the excitation forms a prediction to which the codebook excitation is added. This reduces the amount of information that the codebook excitation has to carry, viz it carries information about changes to the excitation rather than its absolute value.
One difficulty with the apparatus of FIG. 1 is that the temporal resolution of the long term predictor is limited to an integer multiple of the sampling rate.
One prior proposal for alleviating this difficulty involves upsampling the speech signals prior to long-term prediction to increase the resolution of the prediction delay parameter, which however increases the complexity of the apparatus. Another approach is to provide the delay 4 with several taps, each with its own gain factor, a combination of gain factors being chosen from a codebook of gain combinations. This however involves a lengthy search procedure since each delay/gain combination must be tested in the coder to determine the optimum combination.